1. Field of the Invention
The present invention relates to a surface acoustic wave device having an IDT (Inter Digital Transducer) electrode and the like on a piezoelectric substrate. The surface acoustic wave device is used for a surface acoustic wave filter or the like in mobile communications equipment.
2. Description of Related Art
In recent years, a large number of surface acoustic wave devices have been used as constituent elements such as a filter, a delay line, and an oscillator in communications equipment. Particularly, a surface acoustic wave filter that is compact and lightweight and has a high steep cutoff performance as a filter has been frequently used as a filter particularly in an RF stage of a portable terminal device in a mobile communication field. A surface acoustic wave filter having a low loss, a wide bandwidth, and high out-of-band attenuation characteristics has been required.
Single crystals such as a lithium tantalate single crystal (LiTaO3) and a lithium niobate single crystal (LiNbO3) are generally used for a piezoelectric substrate for IDT electrode formation employed for a surface acoustic wave filter used for an RF stage such as a cellular phone. Since the single crystal materials have a higher electromechanical coupling coefficient than those of lithium tetraborate (Li2B4O7) single crystal, a crystal substrate, and so forth, high attenuation characteristics can be obtained in a wide band.
The problem in a case where a surface acoustic wave device is manufactured on a piezoelectric substrate is that when the piezoelectric substrate is subjected to a rapid temperature change during manufacturing processing, polarization occurs in the piezoelectric substrate, so that charges are separated (called a pyroelectric effect).
Since the conductivity of the piezoelectric substrate is low, the stored charges cannot quickly leak out, so that a potential difference occurs between metalized regions (hereinafter referred to as “electrodes”) adjacent to each other in the surface acoustic wave device. Particularly in the place where a space between electrode fingers of the IDT electrode is narrow, the charges are neutralized by ark discharges.
An electric field produced between the electrodes due to the stored charges mechanically damages the piezoelectric substrate even if no ark discharges are induced. According to examination using an electron microscope, it is confirmed that the piezoelectric substrate is cracked and destroyed at edges of the electrodes by the electric field, and the electrodes and probably the piezoelectric substrate are damaged by induction of ark discharges.
Such a damage changes the propagation of an acoustic wave or a surface acoustic wave into a undesirable state, thereby causing a failure or a reduction in performance of the surface acoustic wave device.
Furthermore, when rapid hysteresis of heat is given in manufacturing steps, a wafer serving as the piezoelectric substrate may adsorb a stage, a conveying jig, or the like in a manufacturing apparatus by generated pyroelectricity.
The piezoelectric substrate may be warped or cracked upon creation of a stress in the piezoelectric substrate. Dirt may adhere to a surface of the piezoelectric substrate by the generated pyroelectricity.
Therefore, devices on various processes have been conventionally proposed in order to prevent such discharge destruction.
A configuration in which a ground electrode of a surface acoustic wave device and an input/output electrode of an IDT electrode are connected to a dicing line has been proposed such that all conductor patterns on a piezoelectric substrate will be at the same potential (see JP, 03-293808(A), for example).
Furthermore, all electrode patterns serving as floating electrodes (electrodes that are not conducted to another conductor portion) are connected to a ground electrode using a high-resistance pattern so that static electricity generated by a pyroelectric effect is discharged to the ground without degrading frequency characteristics.
A specific example of the surface acoustic wave device is one in which a high-resistance pattern 9 composed of a high-resistance thin film formed by a silicon (Si) pattern into which impurities have been doped is connected to all electrode patterns serving as floating electrodes, to discharge static electricity to a ground electrode, as its electrode structure is illustrated in a plan view of FIG. 12 (see JP, 2000-183680(A), for example) In FIG. 12, reference numeral 8 denotes an IDT electrode and a reflector that constitute a surface acoustic wave resonator.
Still another example of the surface acoustic wave device is one using a finely folded pattern having a line width of approximately 1 μm (a meander line) as a high-resistance pattern 10 (see JP, 10-200363(A), for example), as its electrode structure is illustrated similarly in plan view of FIG. 13.
In a method of manufacturing a surface acoustic wave device shown in FIGS. 12 and 13, the step of forming an Si pattern 9 or a high-resistance pattern 10 is required, so that the manufacture thereof becomes complicated, and much time is required until the surface acoustic wave device is completed.
In recent years, a method of increasing a bulk conductivity by heating a lithium niobate single crystal in an atmosphere including reducing gas has been proposed.
Consequently, oxygen ions are discharged from a surface of a lithium niobate material by the reducing gas, so that excessive electrons are left to increase the bulk conductivity of the piezoelectric substrate. The bulk conductivity is increased so that storage of charges in manufacturing steps of the surface acoustic wave device is prevented (see JP, 2004-35396(A), for example). Such a piezoelectric substrate whose bulk conductivity is increased is referred to as a “piezoelectric substrate having weak pyroelectric properties”.
In reduction treatment of the piezoelectric substrate, however, when the conductivity of the piezoelectric substrate is too much reduced because the oxygen ions on the surface of the piezoelectric substrate are too much discharged, the composition on the surface of the piezoelectric substrate greatly varies so that the SAW speed varies.
As a result, the piezoelectric characteristics of the piezoelectric substrate having weak pyroelectric properties are degraded, so that the propagation characteristics of the surface acoustic wave are affected. Therefore, frequency characteristics such as an insertion loss or a pass bandwidth of the surface acoustic wave device may, in some cases, be degraded, so that desired characteristics may not be obtained.
An object of the present invention is to provide a surface acoustic wave device manufactured without causing pyroelectric destruction of a minute electrode by a pyroelectric effect of a piezoelectric substrate, without degrading its frequency characteristics such as an insertion loss or a pass bandwidth, and simply, and a manufacturing method therefor.